For example, honeycomb hole structures made of ceramic and having a plurality of through-holes of a cross-section polygonal shape are conventionally known. Such honeycomb structures are used, for example, in particulate-matter-removing filters such as diesel particulate filters. In a production step of such honeycomb structures, a ceramic raw material powder is formed by extruding to produce a green honeycomb molded body. A part of through-holes in this green honeycomb molded body are closed at an end surface. A honeycomb structure is produced by firing the green honeycomb molded body with closed through-holes. Patent Literature 1 discloses a method for producing such honeycomb structures. In Patent Literature 1, a sealing material is pressed with a piston against one end of a honeycomb structure placed within a cylinder to supply the sealing material to the end of through-holes in the honeycomb structure and to close the through-holes.